Filter apparatus are known for selectively separating and binding biomolecules contained in a solution; see, for example, U.S. Pat. No. 5,124,041. In a preferred method, a binding support membrane in contact with a cut-off filter membrane is used for binding the biomolecules such that the solution will pass through the binding support before passing through the cut-off membrane. However, the media layers are loose and not mounted in a support structure with the results that their handling is more difficult than if they were each mounted in a separate support. The device also lacks the capacity in operation to clean the sample in-line prior to the binding layer.
Dead end filters, commonly available from many sources, are cylindrical housings with filter media permanently sealed at the bottom. The housing fits into the top of a centrifuge tube, and the fluid sample is placed in the housing and spun through the filter into the tube. These devices have the disadvantage that only the filtrate is easily recoverable (not the retentate) and they have only one filtration layer.
Reverse flow devices are centrifugation devices in which the flow of filtrate is counter to the direction of the centrifuge force. See, for example, U.S. Pat. No. 4,522,713. They typically consist of a plunger inside a centrifuge tube. The plunger has filter media sealed on its bottom face. In use, a fluid sample and then the plunger are placed in the centrifuge tube. With the plunger in place, the level of the fluid around the plunger is higher than the level within it. Under the force of centrifugation, the fluid seeks to equalize the level forcing some through the filter. These devices have disadvantages in that they do not completely filter a sample (some unfiltered fluid remains outside the plunger), they are usually single layer devices, and they cannot clean the sample prior to separation.